1. Field of the Invention
The present invention relates to a method and an apparatus for inspection of laminate material and in particular to the visual inspection by scanning for the characterization of laminate material, including fiber composite laminate material, metal laminates, brazed parts, welded parts, sandwich parts to determine layer thickness and layer properties such as fiber content, matrix content, density, void content, and other desired properties related to quality control for real time feedback to manufacturing and archiving for future reference, in particular for the determination of the thickness of each ply, and of the physical properties and of the stacking order and the fiber orientation of a fiber reinforced laminate, and the thickness of each ply of metal and sandwich laminates as well as bonded, welded and brazed joints.
2. Description of the Prior Art
Laminated materials, due to the ability to tailor and optimize the electrical, thermal and mechanical properties of the end product, have and are accelerating in application to items extending from sporting goods to aviation products, aerospace and military items, electronic products, automotive parts, and construction components. For primary structural products for the above mentioned industries the ability of the designer to optimize a reinforcing fiber orientation in multi-layers or plies through the cross section allows achieving very high elected directional strength or stiffness to weight ratios.
Likewise, for components with stringent coefficient of thermal expansion requirements the orientation and ratio of reinforcement to matrix are critical for each individual layer or ply. In the electronics industry applications, such as radar cross section reduction, RF energy absorption, radomes and other electronics components, the precise ratios of reinforcement to matrix as well as layer or ply thickness control are required to achieve the required dielectric constant value or electrical performance.
Many of these applications utilize carbon, graphite, glass, ceramic, aramid, metal or other fibers in a matrix of resin (thermoset or thermoplastic), ceramic, metal or other materials. The reinforcing fibers are plied, layered or stacked in the defined sequence and orientation to achieve the required end product performance. To achieve the end product performance not only the fiber plied or stacked sequence and orientation must be precise but also the thickness and reinforcement to matrix ratio for each unique layer or ply must be precisely as defined at design and maintained during the manufacturing process. As the raw materials or pre-impregnated reinforcing fibers have a tolerance as to the amount of reinforcing fiber per unit area, as well as the amount of matrix material per unit area, the resulting layer or ply thickness and reinforcing fiber to matrix material ratio must be adjusted or controlled by the manufacturing process. Currently, quality control procedures utilize as fabricated product thickness and laboratory destructive testing of a specimen from the end product. This methodology does not verify the precise thickness or the reinforcing fiber to matrix material ratio for each layer or ply within the cross section of the laminated end product. Additionally, the time required to perform these test renders the results useless in real time control of the manufacturing process for end products. The number of layers or plies and the orientation of reinforcing fibers for each layer or ply are verified only by visual inspection during manufacturing. This history record is the stamp on a planning document indicating that the inspector visually verified the process. There is no physical evidence available for future evaluation. In some cases photomicrographs of a cross section for a specimen from the end product is generated by encapsulating the specimen, polishing a surface and producing a photograph with a camera mounted onto a microscope. This process is labor intensive and time consuming. The information availability lag from completion of the end product to the photograph is too long to be a valuable tool to the manufacturing process or real time quality efforts. Additionally, interpretation of the photograph is unstructured and requires considerable evaluation and operator experience to obtain precise results.
These same issues occur with end products constructed of laminated sheets of metal adhesive bonded, metal parts welded or brazed, structures of face skins separated by honeycomb, foam or other materials as well as bonded joints.
Other methods have been described and some utilized to evaluate the reinforcing fiber orientation including burning off the resin matrix and removing layer by layer for visual verification of reinforcing fiber orientation and layer or ply count. (U.S. Pat. No. 5,317,387) Ultrasonic inspection is a common methodology utilized in the industry. Ultrasonic inspection is known to define flaws including foreign materials, delaminations and very high void content areas; however, these ultrasonic methodologies do not address the thickness, reinforcing fiber content, matrix content of each unique layer or ply.
Again, it is noted that to assure the performance of the end product to the design criteria the thickness, reinforcing fiber orientation, reinforcing fiber content, matrix content must be precisely as defined in the design details and specifications for each unique layer or ply of the laminate.
U.S. Pat. No. 6,041,132, issued Mar. 21, 2000 to Isaacs, is for a method of computed tomographic inspection which uses a Euclidian reference ply model that has a corresponding Non-Euclidian ply model, which includes reference model plies to extract intensity data from Euclidian slice data (typically having a pixel format) derived from multiple slice X-ray scans using an X-ray scanning system such as the CT system. The multiple slice data is analyzed to determine intensity values for points corresponding to a subject ply of a corresponding reference model ply. The reference model may be a predetermined model such as a mathematically described CAD model file or based on such a CAD model. A preferred method of the present invention includes a transformation of the CAD model data to register the CAD model data to multiple slice data of a standardized object to generate the reference model. Intensity values preferably gray scale pixel values are assigned to points on the reference ply model from the slice data and displayed as a Non-Euclidian image on a monitor.
U.S. Pat. No. 5,317,387, issued May 31, 1994 to Van Hengel, provides a method for the non-destructive determination of the stacking order and the fiber orientation of a fiber reinforced composite laminate. The method comprises illuminating optically successively a series of spots of a cross sectional surface of the laminate under examination and detecting light radiated from the respective illuminated spots. An electrical output signal relative to the amount of light detected is provided and a characterization of the laminate indicative of the stacking order and fiber orientation is determined from the electrical output signal. An apparatus is provided for carrying out the method of the present invention.
U.S. Pat. No. 5,341,436, issued Aug. 23, 1994 to Scott, claims a real-time radioscopy system that produces an X-ray image of a sample of reinforced composite material or a manufactured part that has been molded from the reinforced composite material. By examining the statistics of the distribution of gray levels within the image, it is possible to measure the local and average reinforcing material content (loading) as well as how well the reinforcing material is distributed (the reinforcing material dispersion). The mean gray level is used to determine the local loading of the reinforcing material, which is measured as a function of position in the sample or part using this technique. In addition, an average value of the loading may be obtained. The standard deviation of the gray level image correlates with the quality of dispersion of the reinforcing material.
U.S. Pat. No. 6,041,020, issued Mar. 21, 2000 to Caron, provides the investigation, development and application of a laser-based ultrasonic inspection system for the problems of evaluating polymer/graphite composite materials. The use of lasers to generate and detect ultrasonic waveforms in materials provides a means to detect material properties remotely. The study consisted of three main aspects: 1) A confocal Fabry-Perot (CFP) based system has been devolved which uses light reflected from the CFP interferometer to derive the ultrasonic signal. This allows higher frequency components of the detected waveforms to be discerned when compared to a CFP-based system using light transmitted through the CFP interferometer. 2) Thermoelastic and ablative laser generation of acoustic pulses in polymer/graphite composite materials has been investigated. Thermoelastic generation of ultrasound occurs when thermal energy deposited by a pulsed laser creates a localized expansion in the material. Ablative generation of ultrasound results from the creation of a plasma above the surface when the laser pulse surpasses an intensity threshold. 3) A novel technique, designated Gas-Coupled Laser Acoustic Detection (GCLAD), has been realized, in which the ultrasonic wave is detected optically after it has been transmitted from sample to air. This technique has the advantage of being independent of surface reflectivity and optical smoothness, and has comparable sensitivity to the CFP-based system.
U.S. Pat. No. 5,963,660, issued Oct. 5, 1999 to Koontz, claims an electronic scanner that has a light source and a light sensitive head is connected via a cable to a computer. The scanner head detects reflected light from the surface of the composite material and generates an electronic representation of the surface. A conventional software driver interprets the scanner output to produce an electronic bit-mapped image. The electronic image is then displayed so gaps are readily visible. The electronic representation is also analyzed to determine the presence of laps and gaps, to measure gap widths, to measure the distance between points on the display, and to determine the percentage of the surface covered by fiber material. The electronic representation may also be stored for later analysis.
U.S. Pat. No. 6,028,910, issued Feb. 22, 2000 to Kirchner, concerns a laminographic apparatus and method for imaging individual layers of a multilayer structure, for example the individual layers of a composite, with capabilities for imaging in multiple dimensions or along arbitrary surfaces within the space of the structure. A source of radiation and an areal detector are moved relative to a test specimen positioned therebetween such that a magnified two dimensional image of the test specimen is obtained at the detector. A single translational pass of the test specimen through the source/detector combination provides sensitivity to patterns in the test specimen, which have small scale features lying in a direction parallel to the direction of the pass. An image with sensitivity to features in two perpendicular directions is obtained by taking passes in both directions; no mechanical registration between the perpendicular passes being required. To reconstruct a point along the pass, only local mechanical registration (over for example an inch or so) between the source, test specimen and detector is required for each pass; each point is reconstructed from a predetermined number of images taken over the short distance for which local mechanical registration was required. One or more surfaces of the test specimen may be reconstructed using digitized data of the images.
U.S. Pat. No. 5,562,788, issued Oct. 8, 1996 to Kitson, shows a method of and apparatus for detecting flaws on a composite surface laid-up by a fiber placement machine. The invention includes a vision imaging system mounted on the machine so that it has a field of view of the composite tows after they have been compacted by a compaction roller. In one embodiment, the visual imaging system includes a laser analog displacement sensor. The imaging system provides a computer analysis system with data regarding the location of the edges of the individual composite tows. The computer imaging system uses the tow edge location data to compute the location and size of gaps or overlaps between the tows or the presence of foreign material. This information is useful in quality control models to evaluate the manufacturing process or final part quality.
What is needed is a method and apparatus which can quantify, report and archive the data associated with the properties of each unique layer or ply such as but not limited to the reinforcing fiber type, matrix type, thickness, reinforcing fiber content, matrix content, density, void content, adhesive thickness, braze area thickness and area of migration for jointed materials in a structured, non-personnel interpreted, timely and non-labor intense manner consistent with real-time support to the manufacturing environment.